First generation solar cells were made of bulk silicon. Second generation solar cells include thin film solar cells such as silicon-based single-junction and PIN junction solar cells. Amorphous silicon-based PIN junction solar cells have several advantages over bulk silicon solar cells and thin film silicon-based single-junction solar cells. However, such PIN junction solar cells suffer from light-induced degradation (LID) of the intrinsic amorphous silicon layer, which reduces the efficiency of the solar cells and limits their lifespan. In addition, these amorphous silicon-based PIN junction solar cells exhibit lower mobility of charge carriers than crystalline silicon-based solar cells. Finally, the absorption coefficient of amorphous silicon-based PIN junction solar cells is low.
The compound semiconductor, gallium arsenide (GaAs), finds use in a variety of semiconductor devices, including some photovoltaic devices, due to certain properties such as high mobility of charge carriers, drift velocity, direct bandgap of 1.45 eV, high absorption coefficient and efficiencies that are less sensitive to temperature. However, defects in GaAs crystals prevent these properties from being realized and tend to hamper its use. Thus, efforts have focused on forming GaAs of the highest crystalline quality (i.e., single crystalline GaAs) in order to achieve optimal properties. Such efforts can require intensive extraction, refining, and special growth conditions. Also challenging is the formation of high crystalline quality GaAs over substrates having a lattice mismatch and thermal mismatch with GaAs. As a result, use of GaAs in semiconductor devices has been limited to certain types of structures and is often prohibitively expensive.